Using a Novel Chain of Chemical, Crystallographic, and Isotopic Analytical Techniques to Examine the Formation Histories of Features in Chondritic Meteorites and Orbicular Granites

Author

Samuel P. Alpert, The Graduate Center, City University of New YorkFollow

Date of Degree

6-2024

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Denton S. Ebel

Advisor

Michael K. Weisberg

Committee Members

Kieren Howard

Juliane Gross

Subject Categories

Cosmochemistry | Geochemistry | Geology

Keywords

meteoritics, crystallography, thermobarometry, granites, oxygen isotopes, petrology

Abstract

Understanding the chemistry, crystallography, and isotopic variability of minerals allows us to place significant constraints on the formation history of their host rocks. These constraints provide insight into everything from the distribution of water in the solar system to the onset of plate tectonics. Electron beam instruments and ion probes are important tools used by modern geologists to obtain crystallographic, chemical, and isotopic data. Here I present a new workflow, using these instruments, combining wavelength dispersive spectrometry (WDS), backscattered electron (BSE) imaging, machine learning algorithms, electron backscatter diffraction (EBSD), and secondary ion mass spectroscopy (SIMS), to place quantitative constraints on the formational histories of meteoritic and terrestrial rocks.

First, I present analyses of the modal mineralogy of opaque assemblages (OAs) in the unequilibrated ordinary chondrite Semarkona (LL 3.0), published in Alpert et al. (2021). Through a combination of WDS and machine learning code I found that OAs exhibit a bimodal distribution between sulfur-rich and sulfur-poor endmembers. Based on their modal mineralogy, I conclude that the OAs, present in the chondrite matrix, likely formed alongside Type I and II chondrules in the protoplanetary disk.

Second, I present EBSD and SIMS analyses of magnetite rims observed on these matrix OAs. Rims coarsen further from the core of each OA. The largest exterior grains exhibit single crystallographic orientations. KAM maps of the largest grains shows δ¹⁸O = 2.14 ±2.00‰, δ¹⁷O = 5.65 ±1.48‰, and Δ¹⁷O = 4.53 ±1.01‰. I conclude that the magnetite rims were formed via in situ precipitation from a single, large reservoir of O on the Semarkona parent body and that the source for the iron was leached from the host OA core.

In chapter 3 I analyze the Craftsbury, VT orbicular granite, composed of two lithologies with mica orbicules that are unique compared to those of other orbicular granites globally. Based on WDS analyses I found that there is no significant difference in the chemical composition of the phases between orbicule and matrix for either lithology, but that orthoclase in the trondhjemite exhibits higher (>1.5 wt %) Ba content than that of the granodiorite (below detection). I conclude that this is due to a slower cooling rate, which could also explain the larger orbicules.

Recommended Citation

Alpert, Samuel P., "Using a Novel Chain of Chemical, Crystallographic, and Isotopic Analytical Techniques to Examine the Formation Histories of Features in Chondritic Meteorites and Orbicular Granites" (2024). CUNY Academic Works.
https://academicworks.cuny.edu/gc_etds/5910